The present invention is related to filtering means, in particular to composite polymeric fiber filters, and to the technology for their manufacture.
The creation of filtering materials capable of trapping particles of 0.1-10 microns in size and their increasing use is related to increasingly stringent requirements for quality and reliability of manufactured commodities, as well as to the rapid development of modern technology and production processes, such as, but not limited to, electronics, aviation, automobile industry, electrochemical industry, biotechnology, medicine, etc.
The main industrial manufacturing methods for such materials include production from polymer solutions (V. P. Dubyaga et al., Polymer Membranes, xe2x80x9cChemistryxe2x80x9d Publishing House, Moscow, 1981 (in Russian); V. E. Gul and V. P. Dyakonova, Physical and Chemical Principles of Polymer Films Manufacture, xe2x80x9cHigher School Publishing House, Moscow, 1978 (in Russian); German patent DE 3,023,788, xe2x80x9cCationic absorbent for removing acid dyes etc. From waste waterxe2x80x94prepared from aminoplast precondensate and amine-amide compoundxe2x80x9d), from powders and powder polymer composites (P. B. Zhivotinskiy, Porous Partitions and Membranes in Electrochemical Equipment, xe2x80x9cChemistryxe2x80x9d Publishing House, Leningrad, 1978 (in Russian); Encyclopedia of Polymer Science and Engineering, Wiley, N.Y., 1987, Vol. 8 p. 533), from macromonolithic films (I. Cabasso and A. F. Turbak, xe2x80x9cSynthetic membranesxe2x80x9d, Vol. 1, ACS Symposium, Ser. 154, Washington D.C., 1981, p. 267), and from fibers and dispersions of fibrous polymers (T. Miura, xe2x80x9cTotally dry unwoven system combines air-laid and thermobonding technologyxe2x80x9d, Unwoven World Vol. 73 (March 1988) p.46). The latter method is the most widespread, since it facilitates the manufacture of materials with the optimal cost-quality ratio.
Great interest is also being expressed in the extension of the traditional uses of filtering, materials, especially to combination functions of trapping micro-particles in gaseous and liquid media with the adsorption of molecular admixtures, for example, in the removal of mercaptans, as substrate for catalytic reactions, in the enhancement of the bactericidal effect of the filtering material, etc. Fulfillment of these additional functions is possible due to the introduction into the fiber matrix of fillers of some sort or functional groups giving the formation of additional solid phase, i.e., as a result of manufacturing of composite filtering materials.
At present, high efficiency polymeric filtering materials are manufactured from synthetic fibers by means of a technology that is similar in many aspects to the traditional technology applied in the pulp and paper industry. A long fiber thread is cut into pieces of a given length, which are then subjected to some basic and supplementary operations out of more than 50 possibilities, which may include chemical processing for modification of surface properties, mixing with binding and stabilizing compositions, calendaring, drying process, etc. (O. I. Nachinkin, Polymer Microfilters, xe2x80x9cChemistryxe2x80x9d Publishing House, Moscow, 1985 (in Russian), pp. 157-158). The complexity of such a technological process hampers the manufacture of materials with stable characteristics for subsequent exploitation; results in the high cost of manufactured filtering materials; and practically excludes the manufacture of composites with fillers sensitive to moist, thermal processing.
Low efficiency filtering materials (class ASHRAE) are manufactured by melt blow or spun-bonded processes.
There is, however, a method for the manufacture of ultra-thin synthetic fibers (and devices for their production), which facilitates the combination of the process of fiber manufacture with the formation of a microporous filtering material, and thus reduces the number of technological operations, precludes the necessity for aqueous reaction media, and increases the stability of properties of the product being manufactured (see, for example, U.S. Pat. No. 2,349,950). According to this method, known as xe2x80x9celectrocapillary spinningxe2x80x9d, fibers of a given length are formed during the process of polymer solution flow from capillary apertures under electric forces and fall on a receptor to form an unwoven polymer material, the basic properties of which may be effectively changed.
With this method, fiber formation takes place in the gaps between each capillary, being under negative potential, and a grounded antielectrode in the form of a thin wire, i.e., in the presence of a heterogeneous field, being accompanied by corona discharge. However, the process of solvent evaporation takes place very rapidly, and as a result the fiber is subjected to varying electric and aerodynamic forces, which leads to anisotropy along the fiber width and formation of short fibers.
Manufacture of high-quality filtering materials from such fibers is thus impossible because the electric charge of the fibers is low, such that the process of forming the filtering material is not controlled by electrical force and consequently the filtering material is not uniform.
Exploitation of a device for executing the method described above is complicated by a number of technological difficulties:
1. Capillary apertures become blocked by polymer films that form under any deviation from the technological process conditionsxe2x80x94concentration and temperature of solution, atmospheric humidity, intensity of electric field, etc.
2. The presence of a large number of such formations leads to a complete halt of the technological process or drops form as a consequence of the rupture of the aforementioned films.
3. The presence of high intensity electric field in the area of the precipitation electrode limits the productivity of the method.
Therefore, the manufacture of synthetic fibers by this method is possible from only a very limited number of polymers, for example, cellulose acetate and low molecular weight polycarbonate, which are not prone to the defects described above.
It is necessary to take into account the fact that such an important parameter of filtering materials as monodispersity of the pores (and the resultant separation efficiency of the product) has, in this case, a weak dependency on fiber characteristics and is largely determined by the purely probabilistic process of fiber stacking.
Modern filtering materials are subject to strict, frequently contradictory, requirements. In addition to high efficiency of separation of heterogeneous liquid and gas systems, they are required to provide low hydro- (or aero-) dynamic resistance of the filter, good mechanical strength and technical properties (e.g., pleatability), chemical stability, good dirt absorption capacity, and universality of application, together with low cost.
The manufacture of such products is conditional on the use of high-quality long and thin fibers with an isometric cross-section, containing monodispersed pores and exhibiting high porosity. The practical value of this product may be greatly increased as possible applications are expanded due to the formation of additional phases, i.e., in the manufacture of the above-mentioned composite filtering materials.
At present there is a high demand to high efficiency particulate air (HEPA) filters which are defined as capable of filtering out 99.97% of 0.3 xcexcm particulates in air flowing at 5 cm/sec. Such a requirement is met, for example, by glass-fiber based filters, however on the expense of a high pressure drop, in a range of 30-40 mm H2O.
U.S. Pat. Nos. 4,874,659 and 4,178,157 both teach high efficiency particulate air filters capable of filtering out 99.97% of 0.3 xcexcm particulates in air flowing at 5 cm/sec, characterized by lower pressure drop in a range of 5-10 mm H2O. These filters are made of nonwoven web (U.S. Pat. No. 4,874,659) or sliced films (U.S. Pat. No. 4,178,157) made of polyolefines, such as polyethylene or polypropylene, which are partially melted by heating to about 100xc2x0 C. and are thereafter subjected to an immense electrical field which electrically charges the polymer. The result is a filter media, characterized by thick fibers (10-200 xcexcm) in diameter, low porosity and being electrically charged. The latter property, provides these filters with the high efficiency particulate air (HEPA) qualities. However, such filters suffer few limitations. First, being based on the electrical charge for effective capture of particulates, the performances of such filters are greatly influenced by air humidity, causing charge dissipation. Second, due to their mode of action and to being relatively thin, such filters are characterized by low dust load (the weigh of dust per area of filter causing a two fold increase in pressure drop) per filter weight per area ratio of about 0.8, wherein typically the dust load of such filters is about 50-80 g/m2 and their weight per area is about 80-130 g/m2.
Therefore, the main objective of the proposed technical solution is removal of the above-listed defects of known solutions for filtering applications (primarily directed at the manufacture of microfilters from polymer fibers) and other purposes, including application as micro-filtering means, i.e., the creation of means and the meeting of the above-listed requirements for technical means for the manufacture of micro-filtering materials with new consumer properties.
According to one aspect of the present invention there is provided a device for transforming a liquefied polymer into a fiber structure, including (a) a substantially planar precipitation electrode; (b) a first mechanism for charging the liquefied polymer to a first electrical potential relative to the precipitation electrode; (c) a second mechanism for forming a surface on the liquefied polymer of sufficiently high curvature to cause at least one jet of the liquefied polymer to be drawn by the first electrical potential to the precipitation electrode; wherein the first and second mechanisms are designed such that when a plurality of fibers are precipitated on the precipitation electrode, a high efficiency particulate air unwoven fiber structure, capable of filtering out 99.97% of 0.3 xcexcm particulates in air flowing at 5 cm/sec is obtainable.
According to further features in preferred embodiments of the invention described below, the first mechanism for charging the liquefied polymer to a first electrical potential relative to the precipitation electrode includes in combination (i) a source of high voltage; and (ii) a charge control agent mixed with the liquefied polymer.
According to still further features in the described preferred embodiments the first mechanism for charging the liquefied polymer to a first electrical potential relative to the precipitation electrode further includes (iii) a source of ionized air being in contact with the liquefied polymer.
According to still further features in the described preferred. embodiments the second mechanism is effected by at least one rotating wheel having a rim formed with a plurality of protrusions.
According to still further features in the described preferred embodiments each of the protrusions is formed with a liquefied polymer collecting cavity.
According to still further features in the described preferred embodiments each of the at least one wheel is tilted with respect to the precipitation electrode.
According to still further features in the described preferred embodiments each of the at least one wheel includes a dielectric core.
According to still further features in the described preferred embodiments the second mechanism is effected by a gas bubbles generating mechanism.
According to still further features in the described preferred embodiments the second mechanism is effected by a rotating strap formed with a plurality of protrusions.
The basic device of the present invention includes a grounded moving belt that acts as a precipitation electrode, and an electrode-collector for charging a polymer solution negatively with respect to the moving belt and for producing areas of high surface curvature in the polymer solution.
In one embodiment of the device, the areas of high surface curvature are formed by forcing the polymer solution through a bank of nozzles. The nozzles of the electrode-collector are inserted lengthwise in cylindrical holes sited at intervals in a negatively charged cover plate of the electrode-collector. The source of solvent vapors is connected to the holes. In an alternative configuration, the nozzles are connected by a system of open channels to the solvent vessel.
In one of the implementations, the device is provided with an additional grounded electrode (or alternatively an under potential electrode, of the same polarity of the high voltage electrode, but with lower voltage) which is placed in parallel to the surface of the nozzles of the electrode-collector and which is able to move in the direction normal to the plane of the electrode-collector""s nozzles.
In order to improve the manufacturing process, the additional electrode may take the form of a single wire stretched over the inter-electrode space.
The additional electrode may also take the form of a perforated plate with flange, in which case the surface of the additional electrode, the flange, and the electrode-collector form a closed cavity, and the apertures of the perforated plate are co-axial to the apertures of electrode-collector.
Preferably, a device of the present invention also includes an aerosol generator, made in the form of a hollow apparatus (fluidized bed layer) divided into two parts by a porous electro-conducting partition, which is connected to a mainly positive high-voltage source. The lower part of the cavity forms a pressure chamber, which is connected to a compressor, and the upper part of the cavity is filled with the dispersible filler, for example, polymer powder.
Alternatively, the aerosol generator may be made in the form of a slot sprayer, connected to a positive high-voltage source and a dry fluid feeder, provided with an ejector for supplying powder to the sprayer.
Secondly, the objective put forward in the current invention is obtained by the suggested method of manufacturing of a composite filtering material, stipulating the following operations (stages) (a) preparation of a polymer solution from a polymer, an organic solvent and solubilizing additives, for example, by mixing at elevated temperatures; (b) pouring the polymer solution into the electrode-collector and introducing the dispersible filler, for example, from a polymer of the same chemical composition as that in the solution, into the cavity of electrified aerosol generator; (c) supply of negative high voltage to the electrode-collector, and creation of hydrostatic pressure to facilitate ejection of the polymer solution through the electrode-collector nozzles to produce polymer fibers with a negative electric charge; (d) transfer of the aforementioned fibers under the action of electric and, inertial forces to the precipitation electrode and chaotic stacking of the fibers on its surface to transform the fibers into an unwoven polymer material; (e) displacement of above-described polymer material with the help of the precipitation electrode, followed by interaction of the polymer material with the electrified aerosol cloud formed from the dispersible filler in the aerosol generator under positive high voltage and air pressure, accompanied by penetration of the aerosol cloud into the structure of the negatively charged unwoven polymer material to form a homogeneous composite filtering material.
Thus, according to another aspect of the present invention there is provided a method for forming a polymer into a high efficiency particulate air unwoven fiber structure capable of filtering out 99.97% of 0.3 xcexcm particulates in air flowing at 5 cm/sec, comprising the steps of (a) liquefying the polymer, thereby producing a liquefied polymer; (b) supplementing the liquefied polymer with a charge control agent; (c) providing a precipitation electrode; (d) charging the liquefied polymer to a first electrical potential relative to the precipitation electrode; and (e) forming a surface on the liquefied polymer of sufficiently high curvature to cause at least one jet of the liquefied polymer to be drawn to the precipitation electrode by the first electrical potential difference, thereby forming the unwoven fiber structure capable of filtering out 99.97% of 0.3 xcexcm particulates in air flowing at 5 cm/sec on the precipitation electrode.
According to further features in preferred embodiments of the invention described below, the liquefying is effected by dissolving the polymer in a solvent, thereby creating a polymer solution.
According to still further features in the described preferred embodiments the method further comprising the step of (f) providing vapors of the solvent proximate to the surface of high curvature.
According to still further features in the described preferred embodiments the charge control agent is selected from the group consisting of biscationic amides, phenol and uryl sulfide derivatives, metal complex compounds, triphenylmethanes, dimethylmidazole and ethoxytrimethylsians.
According to still further features in the described preferred embodiments the forming of the surface of high curvature is effected by causing the liquefied polymer to emerge from a nozzle, the surface of high curvature being a meniscus of the liquefied polymer.
According to still further features in the described preferred embodiments the forming of the surface of high curvature is effected by wetting a protrusion having a tip with the liquefied polymer, the surface of high curvature being a surface of the liquefied polymer adjacent to the tip.
According to still further features in the described preferred embodiments the method further comprising the step of (f) moving the precipitation electrode so that the unwoven fiber structure is formed on the precipitation electrode as a sheet.
According to still further features in the described preferred embodiments the method further comprising the step of (f) vibrating the surface of high curvature.
According to still further features in the described preferred embodiments the vibrating is effected at a frequency between about 5000 Hz and about 30,000 Hz.
According to still further features in the described preferred embodiments charging the liquefied polymer to a first electrical potential relative to the precipitation electrode is followed by recharging the liquefied polymer to a second electrical potential relative to the precipitation electrode, the second electrical potential is similar in magnitude, yet opposite in sign with respect to first electrical potential. Preferably the s charge is oscillated between the first and second electrical potentials in a frequency of about 0.1-10 Hz, preferably about 1 Hz.
According to still further features in the described preferred embodiments the method further comprising the steps of (f) charging a filler powder to a second electrical potential relative to the collection surface, the second electrical potential being opposite in sign to the first electrical potential, thereby creating a charged filler powder; and (g) exposing the unwoven fiber structure on the precipitation electrode to the charged powder, thereby attracting the charged filler powder to the unwoven fiber structure.
According to still further features in the described preferred embodiments the method further comprising the steps of (f) supplementing the liquefied polymer with an additive selected from the group consisting of a viscosity reducing additive, a conductivity regulating additive and a fiber surface tension regulating additive.
According to still further features in the described preferred embodiments the viscosity reducing additive is polyoxyalkylein, the conductivity regulating additive is an amine salt and the fiber surface tension regulating additive is a surfactant.
According to still further features in the described preferred embodiments the liquefied polymer is charged negatively relative to the precipitation electrode and wherein the charged powder is charged positively relative to the precipitation electrode.
According to still another aspect of the present invention there is provided a high efficiency particulate air filter comprising unwoven fibers of a polymer, the filter being capable of filtering out at least 99.97% of 0.3 xcexcm particulates in air flowing at 5 cm/sec and having a pressure drop of about 0.75 mm H2O to about 13 mm H2O.
According to still further features in the described preferred embodiments the filter is substantially electrically neutral.
According to still further features in the described preferred embodiments the, fibers have a diameter of about 0.1 xcexcm to about 10 xcexcm
According to yet another aspect of the present invention there is provided a high efficiency particulate air filter comprising unwoven fibers of a polymer, the filter being capable of filtering out at least 99.97% of 0.3 xcexcm particulates in air flowing at 5 cm/sec and having a pressure drop of about 0.75 mm H2O to about 13 mm H2O, wherein at least about 90% of the fibers having a diameter in a range of X and 2X, where X is in a range of about 0.1 xcexcm and about 10 xcexcm.
According to still another aspect of the present invention there is provided a high efficiency particulate air filter comprising unwoven, fibers of a polymer, the filter being capable of filtering out at least 99.97% of 0.3 xcexcm particulates in air flowing at 5 cm/sec and having a pressure drop of about 0.75 mm H2O to about 13 mm H2O, the filter featuring pores formed among the fibers, wherein at least about 90% of the pores having a diameter in a range of Y and 2Y, where Y is in a range of about 0.2 xcexcm and about 10 xcexcm.